The disclosure relates to systems and methods for determining an eyeglass prescription, in particular for visual aids.
The ametropic human eye has refractive errors that in first approximation can be described in terms of a sphere, a cylinder and an axis orientation. This is based on the assumption that a visual defect can be approximately corrected through a lens with simple surfaces such as toroids and spheres. This approximation is adequate to correct an error in the refraction of light rays that enter the center of the eye pupil.
While it is customary to determine the refractive errors of the human eye by relying on the subjective refraction of the patient under examination when presenting to him a plurality of optotypes through lenses of different refractive power (so-called subjective refraction or manifest refraction), the possibility of measuring the refractive errors of the eye has now been available for several years (objective refraction). Moreover, it is possible to measure the refractive power of the eye over the entire pupil and, hence, in particular also in the peripheral areas of the pupil. The measurable errors include for example spherical aberration, coma, trefoil error, higher orders of spherical aberration, etc. In certain implementations, the objective refraction method is based on determining the wavefront of a propagating light bundle. The functional principal of a wavefront refractor is described in document U.S. Pat. No. 6,382,795 B1, which is herein incorporated by reference and for which features protection may be sought, and also includes a synopsis of a plurality of different variants.
The refractive errors or imaging errors of the human eye can be mathematically described by means of so-called Zernike polynomials. The errors of the eye near the center of the pupil in regard to sphere, cylinder and axis can be described, for example, through second-order Zernike polynomials. These errors are therefore often referred to as second-order errors. The errors far from the center can be described through higher-order Zernike polynomials. Therefore, these errors are in general referred to as higher-order errors. The information gained from a wavefront refractor can be used in the development of improved vision aids or improved eyesight correction methods. A well known example for an eyesight correction method is the procedure of wavefront-guided refractive surgery. In this procedure, a volume of any desired geometry is removed from the surface of the cornea in order to correct refractive errors, including those of a higher order. In general, in order to determine an eyeglass prescription for visual aids, an eye care professional determines several parameters. In the case of spectacle lenses, for example, the most relevant ones are: refractive values, usually given in form of sphere, cylinder and axis; fitting parameters, such as pupil distance, fitting height, pantoscopic angle and others; and near vision addition, for example, in case of progressive lenses. For contact lenses, the set of parameters usually includes at least the refractive values, similar to spectacle lenses, and corneal curvature.
Conventionally, the determination of refractive values involves the use of subjective refraction techniques. Typically, this is performed by establishing a first set of (sphere, cylinder, axis) values as starting point for an optimization. The starting point can be provided, e.g., through retinoscopy (an autorefractor measurement) through measurement of the currently worn spectacle lenses, or other methods. Then, an iterative optimization process is started, in which different refractive corrections, i.e. sets of (sphere, cylinder, axis) values are offered to the patient, until he/she achieves a maximum of visual acuity on an eye chart. Examples for determining a subjective refraction of an eye are provided in document U.S. Pat. No. 8,226,238 B2 which is herein incorporated by reference and for which features protection may be sought.
Although newer and advanced objective refraction techniques are available, they have not achieved widespread adoption because many eye care professionals are reluctant to change from the tried and trusted subjective refraction.
Further, it has been found that current methods for providing objective refraction techniques lead to eyeglass prescriptions that deviate from the ones found by subjective refraction techniques for the same eye. Of course, it is undesirable to provide an eyeglass prescription determined by objective refraction technique that does not comply with the eyeglass prescription found via subjective refractive techniques and, hence, may not be considered most convenient by the patient.